Process of cultivating microalgae and a joint method of same with a denitration process

ABSTRACT

The present invention provides a process of cultivating microalgae and a joint method of same jointed with denitration. During the microalgae cultivation, EM bacteria is added into the microalgae suspension. In the nutrient stream for cultivating microalgae, at least one of the nitrogen source, phosphorus source and carbon source is provided in the form of a nutrient salt. During the cultivation, the pH of the microalgae suspension is adjusted with nitric acid and/or nitrous acid. The joint method includes (1) a step of cultivating microalgae; (2) a separation step of separating a microalgae suspension obtained from step (1) into a wet microalgae (microalgae biomass) and a residual cultivation solution; and (3) a NOx absorbing/immobilizing step of denitrating an industrial waste gas with the residual cultivation solution obtained from step (2). The nutrient stream absorbed with NOx obtained from step (3) is used to provide nitrogen source to the microalgae cultivation of step (1).

TECHNICAL FIELD

The present invention relates to a process of cultivating microalgae anda joint method of same with denitration of an industrial waste gas.

BACKGROUND

Energy source and environment are important challenges encountered byhuman being for sustainable development. On the one hand, fossil energysources are non-renewable, and it is emergent to develop alternativeenergy sources. On the other hand, the waste gas and sewage generatedfrom the consumption of fossil energy sources have been resulting insevere impact on the environment, which need to be solved synthetically.

Microalgae is a widely distributed lower plant comprising a great dealof categories, which converts the optical energy into a chemical energyof carbohydrates, such as fat or starch, by effective photosynthesis,and thus is called as a “sun driven activating factory”. The generationof biological energy and chemicals by microalgae is hopeful to achievethe dual purposes of substituting the fossil energy sources and cleaningthe waste gas and sewage.

In the nature, there is a complicated ecological relationship betweenmicroalgae and bacteria. Some specific microalgae and bacteria maybenefit one another, while some others may inhibit one another. A knowndifficulty of cultivating microalgae is the presence of abundant harmfulbacteria in water and air, which is unfavorable for the growth ofmicroalgae, even resulting in a failed cultivation. When an open systemis used to cultivate microalgae, it is impossible to achieve an asepticstate, then it is under high risk of bacteria contamination. A closedcultivation system with rigorous sterilization can achieve an asepticstate, while for a large scale of microalgae cultivation, it is tooexpensive.

NOx in an industrial waste gas is one of the significant air pollutants.NOx not only creates photochemical fog and acid rain, but also resultsin severe greenhouse effect. NOx is also one of the principleinducements of atmospheric haze. The denitration of an industrial wastegas is thus more and more regarded. The processes of denitrating anindustrial waste gas can be classified into a dry process and a wetprocess. Selective Catalytic Reduction (SCR) and Selective non-CatalyticReduction (SNCR) are conventional dry processes, which both involve highcosts of investment and operation, where NOx is reduced to low valuablenitrogen gas without resourcing NOx. The wet process absorbs NOx in awaste gas and immobilizes it in an absorption solution. Such a processhas low costs of investment and operation, whilst two problems need tobe solved. Firstly, NOx in the industrial waste gas is mainly in theform of NO (generally 90% or more), which is little soluble in water,such that a corresponding means is needed to solve the problem involvingthe solubility of NO. Secondly, nitrous acid or nitrite is generallyunavoidable during the absorption, which is hypertoxicity, such that acorresponding means is needed to the problem involving the separation,re-use or disposal thereof.

On the other hand, nitrogen is one of the nutritive elements consumedmost rapidly and most readily lacking during the growth of microalgae.The consumption of a great amount of nitrogenous fertilizer is costlyfor microalgae cultivation. Therefore, it is desirable to combine thecultivation of microalgae and the denitration of an industrial wastegas, which on one hand can use NOx to provide nitrogenous fertilizer tothe microalgae growth, so as to decrease the cost of microalgaecultivation; while on the other hand can purify the waste gas to reducethe discharge of NOx, benefiting the environment. There are somepublished documents disclosing processes of feeding an industrial wastegas into a cultivating device of microalgae for denitration; however,these processes involve some insoluble problems: (1) the denitration ofan industrial waste gas using microalgae must solve the problemsrestricting the commercialization thereof, such as the illumination andmild climate conditions for cultivating microalgae, while the weatherchange necessarily resulting in varied efficiencies of denitration, suchthat the direct feeding of an industrial waste gas is difficult to matchthe emission operation of the industrial waste gas with the cultivationoperation of microalgae, where the two operations interact therebetweento be insufficient to satisfy the requirement of reducing the emissionfrom an industrial production; (2) nitrogen oxide (NO) is the maincomponent of NOx, while NO is little soluble in water, such that thedirect feeding of industrial waste gas cannot solve the problem of thegreat amount of water-insoluble NO in NOx to be little absorbed.

Abundant of NOx is produced from chemical industry. If microalgae isdesired to immobilize NOx in the industrial waste gas, the immobilizingrate of NOx by microalgae should match the emitting rate of NOx from theindustrial discharge, and the floor space occupied by the microalgaeculturing device should be minimized. Generally, the biomassproductivity of photoautotrophic microalgae isphotoautotrophiccultivation less than 30 g·m⁻²·d⁻¹, which is reduced to less than 10g·m⁻²·d⁻¹ for an outdoor large-scale cultivation. With such a biomassproductivity, the plant for denitration of an industrial waste gas willoccupy a large area. Thus it is necessary to increase the biomassproductivity of microalgae. A heterotrophic or mixotrophic cultivationby adding an organic carbon source is a feasible method of acceleratingthe growth of microalgae; however, after adding the organic carbonsource, the microalgae suspension is quite readily to be polluted byharmful bacteria, resulting in rapid growth of the bacterialsignificantly faster than that of the microalgae, which even causes afailed microalgae cultivation.

A scaled microalgae cultivation needs abundant water. If water is notrecycled, the cultivation is costly. Most of the known categories ofmicroalgae cannot adapt to a high concentration ammonium solution, e.g.,ammonium sulphate which is generally used as an inhibitor of microalgae.Meanwhile, when a nitrate is used to provide a nitrogen source to themicroalgae, it is difficult to recycle water used in the cultivation,because metal ions accumulates in the cultivating water, resulting in anincreased salinity, while a high salinity generally inhibits the growthof microalgae.

SUMMARY OF THE INVENTION

The first purpose of the present invention is to increase the biomassproductivity of microalgae, in particular to increase the biomassproductivity of heterotrophic cultivation and mixotrophic cultivation.The second purpose of the present invention is to avoid an asepticoperation during heterotrophic cultivation and mixotrophic cultivation.The third purpose of the present invention is to combine syntheticallythe cultivation of microalgae and the denitration of an industrial wastegas, which can not only use NOx as a nitrogen source for the growth ofmicroalgae, but also avoid any interference caused by the differentoperation conditions between the waste gas emission and the microalgaecultivation. The fourth purpose of the present invention is to use anaqueous solution of nitric acid/hydrogen peroxide as an absorptionsolution for the denitration of an industrial waste gas, so as to avoidgeneration of any toxic nitrous acid, and to increase the availabilityof the hydrogen peroxide during the process.

Specifically, the present invention comprises, for example, thefollowing aspects of contents.

In an aspect, the present invention provides a high efficiency processof cultivating microalgae, characterized in that during the cultivation,EM bacteria is added into the microalgae suspension.

In another aspect, the present invention provides a process ofcultivating microalgae, wherein in the nutrient stream for cultivatingmicroalgae, at least one of the nitrogen source, phosphorus source andcarbon source is provided in the form of a nutrient salt, characterizedin that during the cultivation, the pH of the microalgae suspension isadjusted with nitric acid and/or nitrous acid.

In another aspect, the present invention provides a joint method ofcultivating microalgae and denitrating an industrial waste gas,comprising the steps of:

(1) a cultivation step of cultivating microalgae;

(2) a separation step of separating a microalgae suspension obtainedfrom step (1) into a wet microalgae (microalgae biomass) and a residualcultivation solution;

(3) a NOx immobilizing step of denitrating an industrial waste gas withthe residual cultivation solution obtained from step (2); and

(4) optionally, a drying step of drying the microalgae biomass obtainedfrom step (2) to provide a microalgae product;

wherein a NOx immobilized nutrient stream obtained from step (3) is usedto provide nitrogen source to the microalgae cultivation of step (1).

The step (3) above can be conducted by various ways.

In one preferable embodiment, the joint method involves an acidprocedure, wherein the step (3) comprises:

(i) a sub-step of converting NOx in the industrial waste gas into nitricacid and/or nitrous acid; and

(ii) mixing the residual cultivation solution obtained from step (2)with the nitric acid and/or nitrous acid (preferably the nitric acid andoptionally the nitrous acid) obtained from step (i), so as to achievethe denitration of an industrial waste gas.

In the embodiment, the solution obtained from the mixing is used as theNOx immobilized nutrient stream to provide nitrogen source to themicroalgae cultivation in step (1).

In another preferable embodiment, the joint method involves an alkaliprocedure, wherein the step (3) comprises:

(i′) immobilizing NOx in the industrial waste gas directly with theresidual cultivation solution obtained from step (2).

In the embodiment, the NOx immobilized nutrient stream obtained from thestep (i′) is used to provide nitrogen source to the microalgaecultivation in step (i).

In another aspect, the present invention provides a system useful forthe joint method of cultivating microalgae and denitrating an industrialwaste gas, comprising, optionally from upstream to downstream:

-   -   a NOx immobilizing unit, useful for carrying out the denitration        and providing a NOx immobilized nutrient stream;    -   a microalgae cultivating device, useful for cultivating the        microalgae with the NOx immobilized nutrient stream;    -   a separator, useful for separating a microalgae suspension        obtained from the microalgae cultivating device into a        microalgae biomass and a residual cultivation solution; and    -   a recycle line, useful for recycling the residual cultivation        solution obtained from the separator to upstream of the process,        so as to immobilize the NOx in the industrial waste gas;    -   and optionally, a dryer, useful for drying the microalgae        biomass to provide a microalgae product.

In one preferable embodiment, the NOx immobilizing unit has an inlet forthe NOx-containing industrial waste gas, an inlet for the residualcultivation solution, an outlet for the NOx immobilized nutrient streamand an outlet for the purified industrial waste gas, and optionally aninlet for the nutrient solution.

In one preferable embodiment, the microalgae cultivating device has aninlet for the NOx-immobilized nutrient stream, an inlet for a microalgaestrain, an outlet for the microalgae suspension, and optionally an inletfor the nutrient solution and optionally an inlet for the EM bacteria.

In one preferable embodiment, the separator has an inlet for themicroalgae suspension, an outlet for the microalgae biomass and anoutlet for the residual cultivation solution.

In one preferable embodiment, the recycle line links the outlet for theresidual cultivation solution of the separator to the inlet for theresidual cultivation solution of NOx immobilizing unit.

Preferably, in the method involving the acid procedure, the NOximmobilizing unit or the microalgae cultivating device has an inlet forthe nutrient solution.

Preferably, in the method involving the alkali procedure, the microalgaecultivating device has an inlet for the nutrient solution.

In one preferable embodiment, the joint method involves the acidprocedure, wherein the NOx immobilizing unit comprises a denitrationreactor and a NOx immobilizing nutrient stream formulating device. Inone preferable embodiment, the joint method involves the alkaliprocedure, wherein the NOx immobilizing unit is a denitration reactor.In one preferable embodiment, in the nutrient stream for cultivatingmicroalgae, at least one of the nitrogen source, the phosphorus sourceand the carbon source is provided in the form of a nutrient salt of analkali metal. In one preferable embodiment, during the cultivation,nitric acid and/or nitrous acid is used to adjust the pH of themicroalgae suspension. In one preferable embodiment, in the nutrientstream for cultivating microalgae, the nitrogen source is provided inthe form of an alkali nitrate and/or an alkali nitrite. In onepreferable embodiment, the microalgae is cultivated by heterotrophic ormixotrophic cultivation. Further, in one preferable embodiment, when amicroalgae is cultivated by the heterotrophic or mixotrophiccultivation, the organic carbon source used is at least one selectedfrom the group consisting of sugar, organic acid, salt of an organicacid, alcohol, cellulose hydrolyzate and glucidtemns; preferably atleast one of glucose, fructose, acetic acid, sodium acetate, lacticacid, ethanol, methanol and cellulose hydrolyzate, more preferablyglucose. Further, in one preferable embodiment, when a microalgae iscultivated by the heterotrophic or mixotrophic cultivation, the organiccarbon source used is controlled to have a concentration of 1 g/Lmicroalgae suspension-30 g/L microalgae suspension, preferably 2 g/Lmicroalgae suspension-10 g/L microalgae suspension. In one preferableembodiment, the cultivation is a photoautotrophic cultivation or amixotrophic cultivation, with an illumination intensity of 1000-200000lux. In one preferable embodiment, the process of cultivating microalgaeaccording to the present invention further comprises separating amicroalgae biomass from the microalgae suspension harvested, andrecycling a residual cultivation solution obtained through theseparation of the microalgae biomass to the microalgae cultivation. Inparticular, the prior art deems that nitrate is capable of being used asthe nitrogen source for the microalgae cultivation. However, undercertain situations, the accumulation of metal ions contained in thenitrate may probably inhibit the growth of microalgae. In accordancewith the present invention, when an acid procedure is used, the additionof any extra nitrate is not needed, such that no more metal cations areadditionally introduced, so as not to result in the accumulation ofmetal cations during the process. In one preferable embodiment, duringthe microalgae cultivation, EM bacteria is added into the microalgaesuspension. The EM bacteria is added in an amount of 1×10⁵ cells/Lmicroalgae suspension-9×10⁸ cells/L microalgae suspension, preferably is1×10⁶ cells/L microalgae suspension-5×10⁸ cells/L microalgae suspension,further preferably is 1×10⁶ cells/L microalgae suspension-1×10⁸ cells/Lmicroalgae. In one preferable embodiment, in step (i) of the jointmethod, a wet denitration is used to convert NOx in the industrial wastegas into nitric acid. In one preferable embodiment, the absorptionsolution used to absorb NOx in the wet denitration consists of 0.5 m%-58 m % of nitric acid, 0.001 m %-25 m % of hydrogen peroxide andbalance of water. In one preferable embodiment, the absorption solutionused in the wet denitration consists of 10 m %-25 m % of nitric acid,0.1 m %-1 m % of hydrogen peroxide and balance of water. The presentinvention achieves the following technical effects. According to thepresent invention, during the microalgae cultivation, nitric acid and/ornitrous acid is used to adjust the pH of the microalgae suspension,increasing greatly the microalgae cultivating efficiency. According tothe present invention, the cultivation of microalgae and the denitrationof an industrial waste gas are two relatively independent processes,avoiding the interference caused by the different operation conditionsbetween the waste gas emission and the microalgae cultivation, andavoiding the difficult immobilization as abundant of NO insoluble inwater. So, NOx in the industrial waste gas can provide nitrogen sourceto the microalgae without extra need of additional alkaline solution,which brings lower cost to the cultivating process according to thepresent invention. The present invention avoids the problem caused byaccumulation of metal ions, allowing the cyclic utilization of thecultivating water system. According to the present invention, EMbacteria is added into the microalgae suspension, which can inhibiteffectively the propagation of harmful bacteria, increasingsignificantly the growth rate of microalgae. The advantage of thepresent invention avoids the need of sterilization for a heterotrophiccultivation or a mixotrophic cultivation. According to the presentinvention, hydrogen peroxide with a low concentration and an aqueousnitric acid solution with a low concentration are used as an absorptionsolution for the denitration of an industrial waste gas, where a lowerdecomposition rate and a highly effective availability of hydrogenperoxide are achieved. According to the present invention, a dilutenitric acid is produced simultaneously with the denitration of anindustrial waste gas, which dilute nitric acid is free of toxic nitrousacid and is more preferred to be used as the nitrogen source ofmicroalgae cultivation.

DESCRIPTION OF DRAWINGS

FIG. 1 represents a curve showing the growth of microalgae byphotoautotrophic cultivation.

FIG. 2 represents a curve showing the growth of microalgae bymixotrophic cultivation.

FIG. 3 represents a curve showing the growth of microalgae using anitrate as the nitrogen source.

FIGS. 4 and 5 represent curves showing the growth of microalgae withabundant of an organic carbon source.

FIG. 6 represents a flow chart showing a NOx immobilizing process.

FIGS. 7 and 8 represent curves showing the growth of microalgae using aNOx immobilized nutrient stream as the nitrogen source.

FIG. 9 represents a curve showing the growth of microalgae added with EMbacteria under heterotrophic conditions without light.

FIG. 10 represents a curve showing the NOx immobilizing rate over time.

FIG. 11 represents a curve showing the growth of Chlorella sp. underdifferent conditions.

FIG. 12 represents a curve showing the growth of Spirulina sp. underdifferent conditions.

FIG. 13 represents a general flow chart showing a joint method accordingto the present invention.

FIG. 14 represents a flow chart showing a joint method involving an acidprocedure according to the present invention.

FIG. 15 represents a flow chart showing a joint method involving an acidprocedure combined with an additional photoautotrophic cultivationaccording to the present invention. In these figures:

-   1: NOx immobilizing unit;-   1-1: denitration reactor;-   1-2: NOx immobilizing nutrient stream formulating device-   2: microalgae cultivating device;-   3: separator;-   4: dryer;-   5: CO₂ absorbing nutrient stream formulating device;-   6: CO₂ microalgae cultivating device;-   7: CO₂ microalgae cultivation separator-   A: NOx-containing gas;-   B: NOx immobilized nutrient stream;-   C: purified gas;-   D: microalgae strain;-   E: nutrient solution;-   F: residual cultivation solution;-   G: wet microalgae;-   H: microalgae product;-   I: CO₂ absorbing microalgae nutrient solution;-   J: CO₂ absorbing nutrient stream;-   K: CO₂ absorbing microalgae strain;-   L: CO₂; and-   M: CO₂ cultivation microalgae suspension.

EMBODIMENTS

The embodiments of the present invention will be illustrated below,whilst it should be understood that the protection scopes of the presentinvention are not restricted thereto; instead, the protection scopes aredefined by the claims attached. Unless otherwise defined, the scientificand technical terms used in the specification have the meaningsconventionally known by those skilled in the art. For conflictingmeanings of the terms, they shall subject to the definitions by thepresent specification. In accordance with the present invention, forexample, medium/culture medium means an aqueous system used for thegrowth of microalgae therein during a microalgae cultivation, whichcomprises essential nutrient substances for the growth of microalgae,unless specifically designated. In accordance with the presentinvention, for example, nutrient stream means a stream comprising one ormore nutrient sources, such as a nitrogen source, a phosphorus source ora carbon source, used to formulate the medium, unless specificallydesignated. In accordance with the present invention, for example,microalgae suspension means a system formed by adding a microalgae intoa culture medium, unless specifically designated. In accordance with thepresent invention, when a technical solution is defined in an open mode,e.g., by “comprising”, “containing”, or the like, to provide somemembers, those skilled in the art would understand that an embodimentconsisting of, or consisting essentially of, these members can be usedto practice the technical solution obviously. Therefore, those skilledin the art would understand that a technical solution defined in an openmode also encompasses the specific embodiments defined with “consistingof”, or with “consisting essentially of”. In the context of thespecification, any features or technical means not discussedspecifically will be understood with the meanings known in the artwithout any substantive modification, unless otherwise designated.Moreover, any embodiment described in the specification can beassociated freely with one or more other embodiments described in thespecification, and the technical solution or idea formed therefrom isdeemed as a part of the original disclosure or original record, butcannot be considered as a new content not disclosed or expected by thespecification, unless those skilled in the art believe that thecombination is obviously unfeasible. All features disclosed by thespecification can be combined arbitrarily, and the combination should beunderstood as being disclosed by the present invention, unless thoseskilled in the art believe that the combination is obviouslyunreasonable. The numerical points disclosed by the specificationcomprise not only the specifically mentioned individual numbers, butalso the terminals of each numerical ranges, while any of the rangesformed by the combination of the numerical points should be deemed asbeing disclosed or recorded by the specification, regardless of thenumerical pairs of the lower and upper limits of the ranges beingdisclosed one by one specifically or not.

(I) A Process of Cultivating Microalgae

The present invention provides a process of cultivating microalgae,wherein in the nutrient stream for cultivating microalgae, at least oneof the nitrogen source, phosphorus source and carbon source is providedin the form of a nutrient salt, wherein during the cultivation, the pHof the microalgae suspension is adjusted with nitric acid and/or nitrousacid. According to the present invention, the cultivation may be aphotoautotrophic cultivation (under illumination, using only aninorganic carbon source, such as CO₂, for growth), a heterotrophiccultivation (using only an organic carbon source for growth) or amixotrophic cultivation (under illumination simultaneously with using aninorganic carbon source, such as CO₂, and an organic carbon source forgrowth). The growth of microalgae needs essential conditions, forexample, a suitable temperature for the microalgae suspension,sufficient illumination (photoautotrophic or mixotrophic cultivation),sufficient water, CO₂, and nutrient substance provided in the form of anutrient solution, such as nitrogenous fertilizer, phosphate fertilizerand the like, and controlling the dissolved oxygen and pH of themicroalgae suspension within appropriate ranges. Although theseconditions may vary from microalgae to microalgae, the conditions areknown in the art. Generally, culture is carried out at a temperature of15-40° C., preferably 25-35° C.; and the microalgae suspension has a pHof 6-11, preferably 7-9. For a photoautotrophic cultivation or amixotrophic cultivation, a useful illumination intensity is 1000-200000lux, preferably 5000-150000 lux. The inventors has discovered, throughabundant study and experiments, that when microalgae metabolizes any oneof an alkali nitrate, an alkali nitrite, an alkali carbonate, an alkalibicarbonate, an alkali phosphate and an alkali biphosphate, or acombination thereof, the pH of the microalgae suspension increaseswithout adding CO₂ or a pH regulator into the microalgae suspensionduring the microalgae cultivation. In particular, when microalgaemetabolizes an alkali nitrate, an alkali nitrite or a combinationthereof, the pH of the microalgae suspension increases rapidly.Microalgae is generally cultivated at a pH of 6-11. When the culturemedium contains a nutrient substance above, nitric acid and/or nitrousacid is preferably used to adjust the pH of the microalgae suspension inorder to avoid the pH of the culture medium beyond the range allowed bythe growth of microalgae. The present invention has not any specialrestriction on the category of microalgae. A microalgae with high lipidcontent is preferably cultivated according to the present invention,which can not only produce a biological energy sources, but also reducethe waste gas pollution. Although the cost of heterotrophic ormixotrophic cultivation may be increased partly due to the use of anorganic carbon source, the biomass productivity thereof is increasedsignificantly. Accordingly, the subsequent processing procedures can besimplified. If an aseptic cultivation can be avoided, a rigoroussterilization to the system consuming abundant of steam can be avoided,so as to reduce significantly the cultivation cost. According to thepresent invention, it is especially preferable to use microalgaeadaptable to heterotrophic or mixotrophic cultivation, such as Chlorellasp., Scenedesmus sp., Spirulina sp. or Monoraphidium sp. Surprisingly,when these categories of microalgae are cultured by heterotrophic ormixotrophic cultivation, once a certain quantity of EM bacteria isadded, the cultivation can be conducted successfully even withoutsterilization. The growth rate of microalgae is accelerated greatly.Even if water source contains abundant of harmful bacteria and/or thecultivation is carried out at an open place without sealing, the resultin similarly positive. As compared, without adding the EM bacteria, aheterotrophic or mixotrophic cultivation generally fails. According tothe present invention, a heterotrophic or mixotrophic cultivation isconducted with the addition of EM bacteria, preferably withoutsterilization or the addition of a bactericide. EM bacteria (EffectiveMicroorganisms) is known, which consists essentially of tens ofmicroorganism belonging to photosynthetic bacteria, lactobacillus,microzyme, Gram-positive actinomyce, and filamentous. EM bacteria can beformulated according to the teaching from prior art, or can be obtainedcommercially, and be fermented according to the teaching from prior artor the specification of the commercial formulation before use. Accordingto the present invention, the amount of EM bacteria should satisfy theneed of facilitating the growth of microalgae. The amount of EM bacteriacannot be either too few to be effective, or too many to consumeexcessive nutrient substances due to the competition thereof withmicroalgae. Any way of adding EM bacteria (such as an one-time additionor a batched addition) and any amount of EM bacteria are useful, as longas the growth of microalgae can be facilitated. According to the presentinvention, EM bacteria is added in an amount of 1×10⁵ cells/L microalgaesuspension-9×10⁸ cells/L microalgae suspension, preferably is 1×10⁶cells/L microalgae suspension-5×10⁸ cells/L microalgae suspension,further preferably is 1×10⁶ cells/L microalgae suspension-1×10⁸ cells/Lmicroalgae suspension. According to the present invention, when amicroalgae is cultured by heterotrophic or mixotrophic cultivation, theuseful organic carbon source includes, but not limited to, at least oneof sugar, organic acid, salt of an organic acid, alcohol, cellulosehydrolyzate and glucidtemns; such as at least one of glucose, levulose,acetic acid, sodium acetate, lactic acid, ethanol, methanol andcellulose hydrolyzate, preferably glucose. According to the increasingprofile of the biomass of microalgae and the consuming profile of thenutrient substance in the culture medium, the consumed nutrientsubstance should be supplemented in time. According to the presentinvention, the nutrient substance can be supplemented in any ways, suchas a supplement in batches or a continuous supplement, as long as theamount of the nutrient substance added is controlled within anappropriate range. According to the present invention, for heterotrophicor a mixotrophic cultivation, the concentration of an organic carbonsource is generally controlled at 1 g/L microalgae suspension-30 g/Lmicroalgae suspension, preferably 2 g/L microalgae suspension-10 g/Lmicroalgae suspension. The organic carbon source may be added by anone-time addition or a batched addition. According to the presentinvention, in the alkali nutrient salt, the metal ion is sodium and/orpotassium. According to the present invention, the nitrogen source ispreferably an alkali nitrate and/or an alkali nitrite. According to thepresent invention, the phosphorus source is preferably an alkaliphosphate and/or an alkali biphosphate. According to the presentinvention, a part of the carbon source can be an alkali carbonate and/oran alkali bicarbonate. According to the present invention, when aphotoautotrophic cultivation is used, all or most of the carbon sourceis provided in the form of CO₂. According to the present invention, theamount of the nitrogen source, phosphorus source, or carbon source isprovided as known in the art, for example, the amount of nitrogensource, calculated as nitrogen atoms, is 0.1-400 mmol/L, preferably is10-300 mmol/L, still further preferably is 20-200 mmol/L. The processaccording to the present invention further comprises separating amicroalgae biomass from the microalgae suspension, and recycling aresidual cultivation solution obtained through the separation of themicroalgae biomass to cultivate microalgae. (II) A joint method ofcultivating microalgae and denitrating an industrial waste gas Thepresent invention provides a joint method of cultivating microalgae anddenitrating an industrial waste gas, comprising the steps of:

(1) a cultivation step of cultivating microalgae;

(2) a separation step of separating a microalgae suspension obtainedfrom step (1) into a wet microalgae (microalgae biomass) and a residualcultivation solution;

(3) a NOx immobilizing step of denitrating an industrial waste gas withthe residual cultivation solution obtained from step (2); and

(4) optionally, a drying step of drying the microalgae biomass obtainedfrom step (2) to provide a microalgae product;

wherein the NOx immobilized nutrient stream obtained from step (3) isused to provide nitrogen source to the microalgae cultivation of step(1). The step (3) above can be conducted by various ways. The prior artdeems that nitrate is capable of being used as the nitrogen source forthe microalgae cultivation. However, under certain situations, theaccumulation of metal ion contained in the nitrate may probably inhibitthe growth of microalgae. Therefore, in one preferable embodiment, thejoint method according to the present invention involves an acidprocedure, wherein the step (3) comprises:(i) a sub-step of converting NOx in the industrial waste gas into nitricacid and/or nitrous acid; and(ii) mixing the residual cultivation solution obtained from step (2)with the nitric acid and/or nitrous acid (preferably the nitric acid andoptionally the nitrous acid) obtained from step (i), so as to achievedenitration of the industrial waste gas. In the embodiment, the solutionobtained from mixing is used as the NOx immobilized nutrient stream toprovide nitrogen source to the microalgae cultivation in step (1). Inanother preferable embodiment, the joint method involves an alkaliprocedure, wherein the step (3) comprises:(i′) immobilizing NOx in the industrial waste gas directly with theresidual cultivation solution obtained from step (2). In the embodiment,the NOx immobilized nutrient stream is used to provide nitrogen sourceto the microalgae cultivation. Step (1) can be carried out using anyspecific embodiment in the portion of “a process of cultivatingmicroalgae” above, and the features, steps, conditions or a combinationthereof are useful. According to the present invention, the NOx contentin the industrial waste gas is not specifically restricted. In general,the NOx content in an industrial waste gas is from several hundred ppm(volume) to several thousand ppm, for example, from 100 ppm to 5000 ppm.According to the present invention, in the industrial waste gas to betreated, the molar fraction of NO, based on the total amount of NOx, is≥80%. Further, in the industrial waste gas, the molar fraction of NO,based on the total amount of NOx, is ≥90%. According to the presentinvention, in the acid procedure, any known process can be used in step(i) to convert NOx in the industrial waste gas into nitric acid and/ornitrous acid. Some categories of microalgae cannot metabolize NO₂ ⁻.When cultivating these categories of microalgae, it is needed to selectan appropriate process for immobilizing NOx, so as to convert most orall of NOx into NO₃ ⁻. According to the present invention, any processknown to be appropriate is useful, such as an oxidation absorptionprocess using nitric acid/hydrogen peroxide as an absorbent. Accordingto the present invention, it is preferable to cultivate a microalgaecapable of metabolizing both NO₃ ⁻ and NO₂ ⁻, such as the Chlorella sp.,Monoraphidium sp., Scenedesmus sp. or Spirulina sp. selected by thepresent invention, where the problem of converting to NO₂ ⁻ issubstantially avoided. Considering that the nitrogen source is consumedrapidly in some circumstances of microalgae cultivation, an acidprocedure is preferably used for heterotrophic cultivation, and/or anacid procedure is preferably used for Spirulina sp. cultivation.According to the present invention, in one embodiment, a wet denitrationis preferably used in step (i) to convert NOx in the industrial wastegas into nitric acid and. The absorption solution used to absorb NOx inthe wet denitration consists of 0.5 m %-58 m % of nitric acid, 0.001 m%-25 m % of hydrogen peroxide and balance of water. Such an embodimentis thus called as an acid procedure involved joint method. The inventorshave found by research that regarding an acid procedure involved jointmethod, although an aqueous solution either having a high concentrationof nitric acid/a low concentration of hydrogen peroxide or an aqueoussolution having a high concentration of hydrogen peroxide/a lowconcentration of nitric acid can absorb effectively NOx with lowoxidizability, the two methods may both have the defect of a rapiddecomposition and a great dissipation of hydrogen peroxide. In anaqueous solution having a low concentration of hydrogen peroxide/a lowconcentration of nitric acid, the decomposition of hydrogen peroxide isrelatively slow whilst the aqueous solution having a low concentrationof hydrogen peroxide/a low concentration of nitric acid has a very lowabsorbing activity on NOx with low oxidizability. The inventors havediscovered surprisingly by deep study that although the aqueous solutionhaving a low concentration of hydrogen peroxide/a low concentration ofnitric acid shows a very low activity of absorbing NOx with lowoxidizability at the initial stage of cultivation, the absorbingactivity on NOx with low oxidizability of the aqueous solution increasesgradually. After a period (activating stage), the absorbing activity onNOx with low oxidizability of the aqueous solution reaches a stablestage at a high level. Therefore, preferably, in one embodiment, theabsorption solution having a low concentration of hydrogen peroxide/alow concentration of nitric acid according to the present invention issubjected to an activating stage before the use for absorbing NOx.According to the present invention, in the aforementioned wetdenitration, the absorption solution consists preferably of 10 m %-25 m% of nitric acid, 0.1 m %-1 m % of hydrogen peroxide and balance ofwater; more preferably 10 m %-25 m % of nitric acid, 0.2 m %-1 m % ofhydrogen peroxide and balance of water. As stated above, the absorptionsolution having such a composition has a very low denitration activity,which absorption solution can satisfy the requirement by the denitrationof an industrial waste gas only after an activating stage. Theactivating stage comprises: contacting a solution consisting of 10 m%-25 m % of nitric acid, 0.1 m %-1 m % of hydrogen peroxide and balanceof water with a NOx-containing gas, until the denitrating activity ofthe solution does not increase any more, which means the activating stepbeing completed. In the NOx-containing gas, NO occupies a molarfraction, based on the total amount of NOx, of ≥80%. The NOx-containinggas used for activating the absorption solution can be said industrialwaste gas. According to the present invention, in the aforementioned wetdenitration, the denitration can be conducted at a temperature from −10°C. to 40° C., and a pressure of 0.1 Mpa-1 Mpa; preferably at roomtemperature (10° C.-40° C.) and atmospheric pressure. According to thepresent invention, in the wet denitration above, there is not specialrestriction on the contacting way for the contact between the industrialwaste gas and the active absorption solution, such as any one of thefollowing (A), (B), (C) or a combination thereof: (A) dispersing theindustrial waste gas as bubbles in the absorption solution; (B)dispersing the absorption solution as liquid drops in the industrialwaste gas; (C) contacting liquid with the industrial waste gas in theform of a film-like movement. The way (A) is preferably used. Accordingto the present invention, in the wet denitration, one absorption columnor more absorption columns in series can be used; preferably oneabsorption column or 2-3 absorption columns in series. There is notspecial restriction on the form of the absorption column, such as one ora combination of the followings: a tray absorption column, a bubbleabsorption column, a stirring bubble absorption column, a spray columndispersing the absorption solution as liquid drops in a gas phase, apacked absorption column and a falling film absorption column;preferably a bubble absorption column or a stirring bubble absorptioncolumn. When an alkali procedure or an acid procedure is involved, it ispreferable to adjust the pH of the culture medium by microalgaemetabolism in step (1), such that the residual cultivation solutionobtained from step (2) has a pH>8, more preferably a pH of 9-11. Asstated above, when the culture medium of microalgae contains one ofalkali nitrate, alkali nitrite, alkali carbonate, alkali bicarbonate,alkali phosphate and alkali biphosphate or a combination thereof, the pHof the microalgae suspension increases if no or less CO₂ (or a pHregulator) is provided. Using this phenomena, no or less CO₂ (or a pHregulator) can be provided at the late stage during the microalgaecultivation, but causes the microalgae suspension to be alkaline at theterminal of cultivation through metabolism of the microalgae instead.Accordingly, the residual cultivation solution can be separated from themicroalgae cultivation to immobilize NOx in the waste gas or toneutralize the acid liquid after immobilizing NOx, which is subsequentlyused to in turn provide the essential nitrogen source to the microalgaecultivation. Therefore, in one embodiment, the pH of the residualcultivation solution is controlled, by adjusting the amount of CO₂supplied to the microalgae cultivation, to be >8, more preferably 9-11.The inventors have found that the alkaline residual cultivation solutionafter the separation of microalgae can immobilize NOx in a waste gas orto neutralize the acid liquid after immobilizing NOx with a highefficiency, so as to obtain a solution containing NO₃ ⁻ and/or NO₂ ⁻,which solution may be subsequently used to provide directly nitrogensource to a followed batch of microalgae cultivation. After themetabolism of the nitrogen source by microalgae, the followed batch ofmicroalgae suspension becomes alkaline again. As such, a closed recycleis established between the culture medium of a microalgae cultivationand the absorption solution or neutralization solution of an industrialwaste gas denitration, so as to combine synthetically a “microalgaecultivation” and an “industrial waste gas denitration”, which can notonly convert the nitrogen pollutant into a useful biomass throughmicroalgae effectively, but also maintain the “microalgae cultivation”and the “waste gas denitration” as two relatively independent processes,avoiding any unfavorable interaction therebetween. Anabsorption/immobilization process by alkaline solution is known in theart for the denitration of a waste gas. There are a great deal ofresearches about absorbing/immobilizing NOx of waste gas with analkaline aqueous solution. The present invention can use any one of theknown processes. As known in the art, in order to immobilize NOcompletely, an oxidation column is added before an alkaline solutionabsorption column, which oxidizes NO to NO₂ with the oxygen remained inthe waste gas or by adding ozone, so as to provide an optimaloxidizability (a molar ratio of NO₂/NO) to the alkaline solutionimmobilization process. Catalytically oxidizing catalysts useful forvarious cases are known in the art. For example, active carbon, activecarbon fiber, high silica Na-ZSM-5 molecular sieve or pure silica βmolecular sieve can be used as a catalyst to oxidize NO into NO₂ at roomtemperature. According to the present invention, step (i′) uses analkaline solution absorption process to absorb/immobilize NOx, where aresidual cultivation solution obtained from the microalgae cultivationis used as an absorption solution for absorbing/immobilizing NOx of awaste gas. It is noted that a step of extracting nitrate according tothe conventional alkaline solution immobilization process is omitted.Rather, the solution obtained after the immobilization of NOx is useddirectly to provide nitrogen source to the microalgae cultivationaccording to the present invention. According to the present invention,it is preferable to cultivate microalgae capable of metabolizing bothNO₃ ⁻ and NO₂ ⁻, such as the Chlorella sp., Monoraphidium sp.,Scenedesmus sp. or Spirulina sp. selected by the present invention.According to the present invention, a microalgae resistant to a highalkaline environment is preferred, where the pH of the residualcultivation solution to cultivate such a microalgae can be furtherincreased, so as to increase the efficiency of the reaction thereof withnitric acid and/or nitrous acid or of NOx immobilization. The inventorshave selected out, by abundant of tests, a microalgae resistant to ahighly alkaline environment, such as Chlorella sp., Monoraphidium sp.,Scenedesmus sp. or Spirulina sp., which microalgae can grow healthily ata pH of 9-11. According to the present invention, a microalgae capableof increasing rapidly the pH of the microalgae suspension through themetabolism itself without addition of CO₂ is preferred, where theefficiency of microalgae cultivation can be further increased bycultivating such a microalgae. The inventors have selected out, byabundant of tests, microalgaes capable of increasing rapidly the pH ofthe microalgae suspension, such as Chlorella sp., Monoraphidium sp.,Scenedesmus sp. or Spirulina sp., which microalgaes can increase the pHof the microalgae suspension to be 9-11 within 1-24 hours, allowing themicroalgae suspension reacting with nitric acid and/or nitrous acid orabsorbing/immobilizing NOx with a high efficiency. Preferably, in theNOx immobilized nutrient stream obtained from step (i′) to providenitrogen source to the microalgae, the amount of the nitrogen-containingcompound, calculated as nitrogen atoms, is 0.1-400 mmol/L, preferably10-300 mmol/L, still further preferably 20-200 mmol/L. In addition toNOx, an industrial waste gas may also contain other pollutants, such asSOx. Those skilled in the art can determine, through a simple test (forexample, through measuring the immobilizing rate of NOx or measuring thevaried growth rate of microalgae), whether a waste gas comprises, evenin an excessive amount of, a pollutant significantly damaging the jointmethod according to the present invention. The inventors have discoveredthat when the flue gas from an industrial discharge contains a highcontent of SOx, the efficiency of immobilizing NOx by the residualcultivation solution may be reduced. As required, those skilled in theart can reduce the SOx in a waste gas to a level not damagingsignificantly the joint method according to the present invention by aconventional technic means. A flue gas from general industry discharge,especially a coal flue gas, contains abundant of SOx. Therefore,regarding such an industrial waste gas, SOx contained in the industrialwaste gas should be removed before denitrating the waste gas. Accordingto the present invention, the industrial waste gas is free of SOx or hasbeen desulfurized (removed with of SOx in the waste gas). It should beunderstood that the “microalgae cultivation” and “industrial waste gasdenitration” involved in the present invention are two relativelyindependent processes. The CO₂-containing gas is used mainly to providecarbon source to the microalgae growth, which gas is free substantiallyof SOx or NOx. The CO₂-containing gas may be a purified industrial wastegas (removed with SOx and NOx in the waste gas), or an industrial wastegas free of SOx and NOx. The present invention establishes a cycliceconomic pattern of reducing the discharge of pollutant from anindustrial waste gas and producing a microalgae biomass. NOx in a wastegas from an industrial discharge is used as nitrogen source for thenutrient stream, which not only reduces the pollutant discharge, butalso provides a valuable microalgae biomass. In such a cyclic economicpattern, a part of the cost for treating an industrial waste gas isrecovered by the microalgae cultivation, and the discharge of waste gasand waste water, as well as the environmental pollution, by industry arereduced. A closed recycle is thus formed, where only a microalgaebiomass is obtained at the outlet. The joint method according to thepresent invention can also be further associated with an additionalmicroalgae cultivation. For instance, a microalgae is provided at theinitial stage of a joint method, and in particular, additionalmicroalgaes are provided when the microalgae in the joint method aboveneeds supplement. The additional microalgae cultivation can be aseparate process independent from the steps of the microalgaecultivation of the joint method, so as to input microalgae to, forexample, the microalgae cultivating device as needed, see, for example,FIG. 15. The additional microalgae cultivation can also be incorporatedinto the joint method, e.g., downstream the microalgae cultivation stepsstated above. The additional microalgae cultivation can be aphotoautotrophic cultivation, a mixotrophic cultivation and/or aheterotrophic cultivation, as long as the amount of microalgae generatedsatisfies the need of supplementing the joint method. In one embodiment,the additional microalgae cultivation is a photoautotrophic cultivation,carried out by any known process in the art, see, for example, theprocess showed by FIG. 15. The invention has been illustrated by exampleof the joint method involving a NOx-containing industrial waste gas anda microalgae cultivation, whilst those skilled in the art willunderstand that the joint method is also useful for any otherNOx-containing gas requiring denitration, as long as the gas iscompatible with the microalgae cultivation. (III) A system used for thejoint method of cultivating microalgae and denitrating industrial wastegas The present invention provides a system useful for the joint methodof cultivating microalgae and denitrating industrial waste gas,comprising, optionally from upstream to downstream:

-   -   a NOx immobilizing unit, having an inlet for a NOx-containing        industrial waste gas, an inlet for a residual cultivation        solution, an outlet for a NOx immobilized nutrient stream and an        outlet for a purified industrial waste gas, and optionally an        inlet for a nutrient solution, useful for denitration and        providing the NOx immobilized nutrient stream;    -   a microalgae cultivating device, having an inlet for the NOx        immobilized nutrient stream, an inlet for a microalgae strain        and an outlet for a microalgae suspension, and optionally an        inlet for a nutrient solution, optionally an inlet for EM        bacteria, useful for microalgae cultivation using the NOx        immobilized nutrient stream;    -   a separator, having an inlet for the microalgae suspension, an        outlet for a microalgae biomass and an outlet for the residual        cultivation solution, useful for separating the microalgae        suspension obtained from the microalgae cultivating device into        the microalgae biomass and the residual cultivation solution;        and    -   a recycle line, linking the outlet for the residual cultivation        solution of the separator to the inlet for the residual        cultivation solution of the NOx immobilizing unit;    -   and optionally, a dryer, useful for drying the microalgae        biomass to provide a microalgae product. Preferably, for a joint        method involving the acid procedure, the NOx immobilizing unit        has an inlet for the nutrient solution. Preferably, for a joint        method involving the alkali procedure, the microalgae        cultivating device has an inlet for the nutrient solution. In        one preferable embodiment, the joint method involves the acid        procedure, wherein the NOx immobilizing unit comprises a        denitration reactor and a NOx immobilizing nutrient stream        formulating device. In one preferable embodiment, the joint        method involves the alkali procedure, wherein the NOx        immobilizing unit is a denitration reactor. Referring to FIG.        13, an embodiment of the system according to the present        invention comprises: NOx immobilizing unit 1; microalgae        cultivating device 2; separator 3; and dryer 4. Regarding an        acid procedure, NOx immobilizing unit 1 comprises: denitration        reactor 1-1; and NOx immobilizing nutrient stream formulating        device 1-2 (referring to FIG. 14); while regarding an alkali        procedure, 1 is 1-1: denitration reactor. Thus in the system, a        NOx-containing gas A, a residual cultivation solution F from        separator 3 and optionally a nutrient solution E is fed into NOx        immobilizing unit 1, and a NOx immobilized nutrient stream B and        a purified gas C are obtained after treatment; subsequently, the        NOx immobilized nutrient stream B from NOx immobilizing unit 1,        a microalgae strain D and optionally a nutrient solution E are        fed into microalgae cultivating device 2; a microalgae        suspension from the cultivation is fed into separator 3 and        separated therein to provide a wet microalgae (microalgae        biomass) G and a residual cultivation solution F; and the        microalgae biomass G is fed into dryer 4, where it is dried to        provide a microalgae product H. Preferably, for an acid        procedure, the nutrient solution E is added into NOx        immobilizing unit 1. Preferably, for an alkali procedure, the        nutrient solution E is added into microalgae cultivating device        2.

FIG. 14 exemplifies an acid procedure conforming to the embodiment ofFIG. 13. As stated above, for an acid procedure, NOx immobilizing unit 1consists of denitration reactor 1-1 and NOx immobilizing nutrient streamformulating device 1-2. Accordingly, regarding NOx immobilizing unit 1,a NOx-containing gas A and an aqueous solution having a lowconcentration of hydrogen peroxide/a low concentration of nitric acidused as a NOx immobilizing solution (not shown in the figure) is fedinto denitration reactor 1-1, where a NOx immobilized nutrient streamand a purified gas C are obtained after treatment; the NOx immobilizednutrient stream, and a residual cultivation solution F from separator 3and nutrient solution E is fed into NOx immobilizing unit 1, where a NOximmobilized nutrient stream B is obtained after treatment. The otherfacilities and processing procedures are same as the general embodimentshowed by FIG. 13.

FIG. 15 instantiates a combination of the joint method according to thepresent invention with an additional microalgae cultivation. In thecombined process, the joint method according to the present inventionhas processing procedures as showed by FIG. 13, except that themicroalgae strain D fed to microalgae cultivating device 2 comesspecifically from an additional microalgae cultivation process, which isa photoautotrophic cultivation. The additional microalgae cultivationprovides microalgae to the joint method at the initial stage, inparticular providing additional microalgaes when the microalgae in thejoint method above needs supplement. In the combined process showed byFIG. 15, the additional microalgae cultivation can be a separate processindependent from the steps of the microalgae cultivation of the jointmethod, so as to input microalgae to, for example, the microalgaecultivating device as needed

According to the discussion above, the present invention provides in oneaspect the following embodiments:

1. A process of cultivating microalgae, characterized in that, duringthe microalgae cultivation, EM bacteria is added into the microalgaesuspension.

2. The process according to embodiment 1, characterized in that themicroalgae is a heterotrophic or mixotrophic microalgae.

3. The process according to embodiment 2, characterized in that themicroalgae is selected from the group consisting of Cyanophyta andChlorophyta.

4. The process according to embodiment 2, characterized in that themicroalgae is Chlorella sp., Scenedesmus sp., Monoraphidium sp. orSpirulina sp.

5. The process according to embodiment 2, characterized in that theorganic carbon source used is at least one selected from the groupconsisting of sugar, organic acid, salt of an organic acid, alcohol,cellulose hydrolyzate and glucidtemns; preferably at least one ofglucose, levulose, acetic acid, sodium acetate, lactic acid, ethanol,methanol and cellulose hydrolyzate, more preferably glucose.6. The process according to embodiment 2 or 3, characterized in that theconcentration of the organic carbon source used is generally controlledat 1 g/L microalgae suspension-30 g/L microalgae suspension, preferably2 g/L microalgae suspension-10 g/L microalgae suspension.7. The process according to any one of embodiments 1-6, EM bacteria isadded in an amount of 1×10⁵ cells/L microalgae suspension-9×10⁸ cells/Lmicroalgae suspension, preferably is 1×10⁶ cells/L microalgaesuspension-5×10⁸ cells/L microalgae suspension, further preferably is1×10⁶ cells/L microalgae suspension-1×10⁸ cells/L microalgae suspension.8. The process according to any one of embodiments 1-7, characterized inthat the cultivation is conducted at a temperature of 15-40° C., and themicroalgae suspension has a pH of 6-11.9. The process according to any one of embodiments 1-8, characterized inthat when the cultivation is a photoautotrophic cultivation or amixotrophic cultivation, the illumination intensity is 1000-200000 lux.10. The process according to any one of embodiments 1-9, characterizedin that during the cultivation, NO₃ ⁻ and/or NO₂ ⁻ are used as nitrogensource, preferably a nitrate and/or a nitrite obtained from thedenitration of an industrial waste gas being used as nitrogen source.11. The process according to any one of embodiments 1-10, wherein in thenutrient stream for cultivating microalgae, at least one of the nitrogensource, phosphorus source and carbon source is provided in the form of anutrient salt, characterized in that during the culture, the pH of themicroalgae suspension is adjusted with nitric acid and/or nitrous acid.12. The process according to any one of embodiments 1-11, characterizedin that the nitric acid is obtained by converting NOx in an industrialwaste gas into nitric acid through a wet denitration; and the absorptionsolution used in the wet denitration consists of 0.5 m %-58 m % ofnitric acid, preferably 10 m %-25 m % of nitric acid, 0.001 m %-25 m %of hydrogen peroxide, preferably 0.1 m %-1 m % of hydrogen peroxide, andbalance of water.13. A joint method of cultivating microalgae and denitrating anindustrial waste gas, comprising the steps of:(1) a cultivation step of cultivating microalgae;(2) a separation step of separating a microalgae suspension obtainedfrom step (1) into a wet microalgae (microalgae biomass) and a residualcultivation solution;(3) a NOx immobilizing step of denitrating the industrial waste gas withthe residual cultivation solution obtained from step (2); and(4) optionally, a drying step of drying the microalgae biomass obtainedfrom step (2) to provide a microalgae product;wherein a NOx immobilized nutrient stream obtained from step (3) is usedto provide nitrogen source to the microalgae cultivation of step (1).14. The joint method according to embodiment 13, characterized in thatthe step (1) of cultivating microalgae is carried out using a processaccording to any one of embodiments 1-12.15. The joint method according to embodiment 13 or 14, characterized inthat the joint method involves an acid procedure, wherein the step (3)comprises:(i) a sub-step of converting NOx in the industrial waste gas into nitricacid and/or nitrous acid; and(ii) mixing the residual cultivation solution obtained from step (2)with the nitric acid and/or nitrous acid obtained from step (i), so asto achieve denitration of the industrial waste gas.16. The joint method according to embodiment 13 or 14, characterized inthat the joint method involves an alkali procedure, wherein the step (3)comprises:(i′) immobilizing NOx in the industrial waste gas directly with theresidual cultivation solution obtained from step (2).17. The method according to any one of embodiments 13-16, characterizedin that when the cultivation is a photoautotrophic cultivation or amixotrophic cultivation, a CO₂-containing gas is used as an inorganiccarbon source, preferably the CO₂-containing gas being an purifiedindustrial waste gas or an industrial waste gas free of SOx and NOx.18. The method according to any one of embodiments 13-17, characterizedin that in the NOx immobilized nutrient stream, the amount of thenitrogen-containing compound, calculated as nitrogen atoms, is 0.1-400mmol/L, preferably 10-300 mmol/L, still further preferably 20-200mmol/L.19. The method according to any one of embodiments 13-18, characterizedin that the industrial waste gas is free of SOx or is desulfurized.20. The method according to any one of embodiments 13-19, characterizedin that during the late stage of the microalgae cultivation, no or lessCO₂ or a pH regulator is provided, while the microalgae suspension isled to be alkaline at the end of cultivation through the microalgaemetabolism; wherein the alkali nutrient salt is any one of an alkalinitrate, an alkali nitrite, an alkali carbonate, an alkali bicarbonate,an alkali phosphate and an alkali biphosphate, or a combination thereof,preferably an alkali nitrate and/or an alkali nitrite.

In another aspect, the present invention provides the followingembodiments:

1. A process of cultivating microalgae, wherein in the nutrient streamfor cultivating microalgae, at least one of the nitrogen source,phosphorus source and carbon source is provided in the form of anutrient salt, characterized in that during the cultivation, the pH ofthe microalgae suspension is adjusted with nitric acid and/or nitrousacid.2. The process according to embodiment 1, characterized in that, duringthe cultivation, EM bacteria is added into the microalgae suspension.3. The process according to embodiment 2, characterized in that EMbacteria is added in an amount of 1×10⁵ cells/L microalgaesuspension-9×10⁸ cells/L microalgae suspension, preferably is 1×10⁶cells/L microalgae suspension-5×10⁸ cells/L microalgae suspension,further preferably is 1×10⁶ cells/L microalgae suspension-1×10⁸ cells/Lmicroalgae suspension.4. The process according to any one of embodiments 1-3, characterized inthat the microalgae is a heterotrophic or mixotrophic microalgae.5. The process according to any one of embodiments 1-4, characterized inthat the microalgae is a Cyanophyta or a Chlorophyta, preferablyChlorella sp., Scenedesmus sp., Monoraphidium sp. or Spirulina sp.6. The process according to any one of embodiments 1-5, characterized inthat the nitric acid is obtained by converting NOx in an industrialwaste gas into nitric acid through a wet denitration; and the absorptionsolution used in the wet denitration consists of 0.5 m %-58 m % ofnitric acid, preferably 10 m %-25 m % of nitric acid, 0.001 m %-25 m %of hydrogen peroxide, preferably 0.1 m %-1 m % of hydrogen peroxide, andbalance of water.7. The process according to embodiment 4, characterized in that theorganic carbon source used is at least one selected from the groupconsisting of sugar, organic acid, salt of organic acid, alcohol,cellulose hydrolyzate and glucidtemns.8. The process according to embodiment 4, characterized in that theorganic carbon source is used at a concentration of 1 g/L microalgaesuspension 30 g/L microalgae suspension.9. The process according to embodiment 1, characterized in that when thecultivation is a photoautotrophic cultivation or a mixotrophiccultivation, the illumination intensity is 1000-200000 lux.10. A joint method of cultivating microalgae and denitrating anindustrial waste gas, comprising the steps of:(1) a cultivation step of cultivating microalgae;(2) a separation step of separating a microalgae suspension obtainedfrom step (1) into a wet microalgae (microalgae biomass) and a residualcultivation solution;(3) a NOx immobilizing step of denitrating the industrial waste gas withthe residual cultivation solution obtained from step (2), comprising:(i) a sub-step of converting NOx in the industrial waste gas into nitricacid and/or nitrous acid; and(ii) mixing the residual cultivation solution obtained from step (2)with the nitric acid and/or nitrous acid obtained from step (i), so asto achieve denitration of the industrial waste gas;(4) optionally, a drying step of drying the microalgae biomass obtainedfrom step (2) to provide a microalgae product;wherein a NOx immobilized nutrient stream obtained from step (3) is usedto provide nitrogen source to the microalgae cultivation of step (1).11. The method according to embodiment 10, characterized in that in step(2), NOx in the industrial waste gas is converted into nitric acidthrough a wet denitration; and the absorption solution used in the wetdenitration consists of 0.5 m %-58 m % of nitric acid, preferably 10 m%-25 m % of nitric acid, 0.001 m %-25 m % of hydrogen peroxide,preferably 0.1 m %-1 m % of hydrogen peroxide, and balance of water.12. The joint method according to embodiment 10 or 11, characterized inthat the step (1) of cultivating microalgae is carried out using theprocess according to any one of embodiments 1-9.13. The joint method according to any one of embodiments 10-12,characterized in that in the nutrient stream of step (1), the nitrogensource is provided in the form of an alkali nitrate and/or an alkalinitrite.14. The joint method according to any one of embodiments 10-13,characterized in that the joint method further comprises an additionalmicroalgae cultivation step, which provides microalgae at the initialstage of the joint method, and/or provides supplementary microalgae whenthe microalgae in the microalgae cultivation step (1) needs supplement.15. The joint method according to embodiment 14, characterized in thatthe additional microalgae cultivation step is a separate processindependent from the microalgae cultivation step (1), so as to inputmicroalgae to the microalgae cultivation step (1) as needed16. The joint method according to embodiment 14, characterized in thatthe additional microalgae cultivation step is incorporated into thejoint method, and is placed upstream of the microalgae cultivation step(1).17. A system useful for the joint method of cultivating microalgae anddenitrating industrial waste gas, comprising, optionally from upstreamto downstream:

-   -   a NOx immobilizing unit, useful for carrying out the denitration        and providing a NOx immobilized nutrient stream;    -   a microalgae cultivating device, useful for cultivating        microalgae with the NOx immobilized nutrient stream;    -   a separator, useful for separating a microalgae suspension        obtained from the microalgae cultivating device into a        microalgae biomass and a residual cultivation solution; and    -   a recycle line, useful for recycling the residual cultivation        solution obtained from the separator to upstream of the process,        so as to immobilize NOx in the industrial waste gas;    -   and optionally, a dryer, useful for drying the microalgae        biomass to provide a microalgae product.        18. The system according to embodiment 17, wherein the NOx        absorbing unit has an inlet for a NOx-containing industrial        waste gas, an inlet for the residual cultivation solution, an        outlet for the NOx immobilized nutrient stream and an outlet for        the purified industrial waste gas;    -   the microalgae cultivating device has an inlet for the NOx        immobilized nutrient stream, an inlet for a microalgae strain        and an outlet for the microalgae suspension;    -   the separator has an inlet for the microalgae suspension, an        outlet for the microalgae biomass and an outlet for the residual        cultivation solution; and    -   the recycle line links the outlet for the residual cultivation        solution of the separator to the inlet for the residual        cultivation solution of the NOx absorbing unit.        19. The system according to embodiment 18, wherein the NOx        absorbing unit comprises a denitration reactor and a NOx        immobilizing nutrient stream formulating device.        20. The system according to embodiment 18 or 19, characterized        in that the system further comprises an additional microalgae        cultivation device, which provides microalgae to the system at        the initial stage of the joint method, and/or provides        supplementary microalgae when the microalgae in the microalgae        cultivation device needs supplement.

EXAMPLES

The present invention will be further illustrated by examples below.

Measurement of optic density of the microalgae suspension (OD₆₈₀ value):measured by a spectrophotometry, using distilled water as control, andmeasuring the optic absorption by the microalgae suspension at awavelength of 680 nm, which was used as an indicator of the microalgaeconcentration.

Measurement of nitrogen content of a solution: using an ionchromatograph, Model ICS3000 (Dionex company, USA) to measure the NO₃ ⁻content or NO₂ ⁻ content in an aqueous solution, which chromatograph wasequipped with an EG40 eluent generator, an electrical conductivitydetector and a chameleon chromatogram workstation; Model ionpac AS11-HCseparating column (250 mm×4 mm i.d.); Model ionpac AG11 guard column (50mm×4 mm i.d.); ASRS-ULTRA anion self-generating suppressor. Eluent: KOHsolution; with a flow rate of 1 ml/min; an eluent concentration of 30mmol/L; a feeding volume of 60 μl; a column temperature of 30° C.; asuppression current of 100 ma; an external standard method forquantifying peak area.

Count of bacteria: carried out according to the steps of:

1. Washing the sample: taking 1 ml of the sample, and washing 2-3 timeswith 1×PBS;

2. Separating preliminarily: centrifuging at 1000 rpm for 2 min usingdifferent centrifugal forces between microalgae and bacteria, toseparate preliminarily out the microalgae (bacteria being in thesupernatant, while microalgae being precipitated); and optionallyrepeating this step for a higher microalgae content;3. Collecting the supernatant, wherein the quantity of microalgae in thesupernatant was ignorable, centrifuging at 8000 rpm for 5 min, anddisposing the supernatant;4. Resuspending the precipitation with 500 ul of bacteria membranepermeabilizer, and reacting at room temperature for 15 min;5. Centrifuging at 8000 rpm for 5 min, and washing the bacteria solution2 times with 1×PBS;6. Resuspending the bacteria by adding 100 ul 1×PBS, and adding a stocksolution of 5 ul PI staining solution, for reaction at room temperaturefor 30 min;7. Observing and counting the bacteria under a fluorescence microscope,wherein the maximum bacteria quantity in 4 big grids was restricted tobe 1000, and when the maximum quantity was greater than 1000, dilutingthe bacteria solution for re-counting.8. Calculation equation:Bacteria density in the solution measured=counting result/4×dilutionfold×4×10⁴/mlThe Main Reagents:

Reagents used Manufacturer PI Viability Cat No. FXP002, StainingSolution Beijing 4A Biotech Co., Ltd, China. Membrane Cat No. REK3004,permeabilizer REAL_AB company, Tianjin, China. Phosphate buffer Cat No.REK3013, (10 × PBS, REAL_AB ph7.4, cell culture company, Tianjin, level,aseptic) China. Cell climbing slice NESTMain Instruments:

Instruments used Manufacturer Counting plate Shanghai PrecisionInstruments, Co., Ltd., China Fluorescence Olympus BX-51 microscope

Culture medium for microalgae: the ingredients of the culture mediumwere showed in Table 1-Table 5.

In the present invention, an activity of denitration denoted a molarratio of an NOx content in an industrial waste gas after treatment to anNOx content in the industrial waste gas before treatment.

TABLE 1 culture medium BG11 Composition, Ingredients mg/L K₂HPO₄•3H₂O 40NaNO₃ 1500 Na₂CO₃ 20 MgSO₄•7H₂O 75 CaCl₂•2H₂O 36 Citric acid 6 Ferricammonium citrate 6 Disodium EDTA 1 Trace element A5 (Table 2) 1

TABLE 2 trace element A5 Composition, Ingredients mg/L H₃BO₃ 2860MnCl₂•4H₂O 1810 ZnSO₄•7H₂O 222 CuSO₄•5H₂O 79 NaMoO₄•5H₂O 390Co(NO₃)₂•6H₂O 50

TABLE 3 Z-medium composition, Ingredients g/L KH₂PO₄•3H₂O 0.50 NaNO₃ 2.5NaHCO₃ 16.8 NaCl 1.0 MgSO₄•7H₂O 0.20 K₂SO₄ 1.0 CaCl₂•2H₂O 0.04FeSO₄•7H₂O 0.01 Disodium EDTA 0.08 Trace element 1 ml A5 (Table 2)

TABLE 4 heterotrophic cultivation medium Composition, Ingredients g/LKNO₃ 10 Na₂HPO₄•12H₂O 8.8 KH₂PO₄ 0.3 MgSO₄•7H₂O 0.2 CaCl₂•2H₂O 0.02Fe-EDTA solution 1 ml Trace element (Table 5) 3.5 ml Fe-EDTA solution:15 g/L and FeSO₄•7H₂O EDTA 1.4 g/L

TABLE 5 trace element Composition, Ingredients g/L H₃BO₃ 2.86 MnCl₂•4H₂O0.11 ZnSO₄•7H₂O 9.22 CuSO₄•5H₂O 1.00 (NH₄)₆Mo₇O₂₄•4H₂O 0.10Co(NO₃)₂•6H₂O 0.90

Example 1

The example illustrated the impact of the addition of EM bacteria on aphotoautotrophic cultivation.

A BG11 medium (having nutrient ingredients according to Table 1, withoutsterilization) was used to cultivate Chlorella sp., with a temperaturecontrolled between 20 and 30° C. Compressed air and CO₂ were fed forcultivation. When the microalgae suspension had a pH>10, CO₂ was fed,while when the microalgae suspension had a pH<7.5, the feeding of CO₂was ceased. Natural sunlight was used for cultivation. The illuminationintensity at daytime was controlled up to 60000 lux. The OD₆₈₀ value ofthe microalgae suspension was detected every day. Harvest was made aftera 14 day continuous cultivation. The feeding of CO₂-containing mixed gaswas ceased 1 day before the cultivation terminal. Then, a microalgaebiomass and a residual cultivation solution were obtained throughcentrifugal separation. The growth curve of the microalgae was showed inFIG. 1. The two tests in FIG. 1 were substantially same, except that oneof the both tests was not added with EM bacteria, whilst another wasadded with EM bacteria in an amount of 3.6×10⁶ cells/L microalgaesuspension. Regarding the test with the addition of EM bacteria, duringthe cultivation, the bacteria count of the microalgae suspensionmonitored was <6.7×10⁶/ml microalgae suspension. At the cultivationterminal, the pH of the microalgae suspension had increased naturally to9.8. It could be seen from FIG. 1 that under photoautotrophiccultivation conditions, the addition of EM bacteria promoted the growthof microalgae.

Examples 2-5 illustrated the impact of the amount of EM bacteria addedon the microalgae cultivation for a mixotrophic cultivation.

Example 2

A BG11 medium (having nutrient ingredients according to Table 1, withoutsterilization) was used to cultivate Chlorella sp., with addition of 2g/L glucose During the culture, at a temperature controlled between 20and 30° C. Compressed air and CO₂ were fed for cultivation. When themicroalgae suspension had a pH>10, CO₂ was fed, while when themicroalgae suspension had a pH<7.5, the feeding of CO₂ was ceased.Natural sunlight was used for cultivation. The illumination intensity atdaytime was controlled up to 60000 lux. The OD₆₈₀ value of themicroalgae suspension was detected every day. The growth curve of themicroalgae was showed in FIG. 2. The EM was added in an amount of3.6×10⁶ cells/L microalgae suspension. During the cultivation, thebacteria count of the microalgae suspension monitored was <8×10⁶/mlmicroalgae suspension. Harvest was made after a 14 day continuouscultivation. The feeding of CO₂-containing flue gas was ceased one daybefore the cultivation terminal, and the pH of the microalgae suspensionwas allowed to increase to 9.4. Then, a microalgae biomass and aresidual cultivation solution were obtained through centrifugalseparation.

Example 3

The example was substantially same as example 2, except that EM wasadded in an amount of 1.8×10⁷ cells/L microalgae suspension. After theaddition of EM, during the stable state of the cultivation, the bacteriacount of the microalgae suspension monitored was <1×10⁷/ml microalgaesuspension. At the cultivation terminal, the pH of the microalgaesuspension had increased naturally to 9.3. The growth curve of themicroalgae was showed in FIG. 2.

Example 4

The example was substantially same as example 2, except that EM wasadded in an amount of 3.6×10⁷ cells/L microalgae suspension. After theaddition of EM, during the stable state of the cultivation, the bacteriacount of the microalgae suspension monitored was <2×10⁷/ml microalgaesuspension. At the cultivation terminal, the pH of the microalgaesuspension had increased naturally to 8.9. The growth curve of themicroalgae was showed in FIG. 2.

Example 5

The example was substantially same as example 2, except that EM wasadded in an amount of 7.2×10⁷ cells/L microalgae suspension. During thecultivation, the bacteria count of the microalgae suspension monitoredwas <5.8×10⁷/ml microalgae suspension. At the cultivation terminal, thepH of the microalgae suspension had increased naturally to 8.7. Thegrowth curve of the microalgae was showed in FIG. 2.

Comparative Example 1

The example was substantially same as example 2, except that no EMbacteria was added.

During the cultivation, the bacteria count of the microalgae suspensionmonitored was up to 1.2×10⁸/ml microalgae suspension. At the cultivationterminal, the pH of the microalgae suspension had increased naturally to7.9. The growth curve of the microalgae was showed in FIG. 2.

It could be seen from FIG. 2 that under mixotrophic cultivationconditions, the addition of EM bacteria promoted the growth ofmicroalgae.

Examples 6-8 illustrated the metabolism of nitrate and nitrite bymicroalgae.

Example 6

A BG11 medium (having nutrient ingredients according to Table 1, withoutsterilization) was used to cultivate Chlorella sp., at a temperaturecontrolled between 20 and 30° C. Compressed air and CO₂ were fed forcultivation. When the microalgae suspension had a pH>10, CO₂ was fed,while when the microalgae suspension had a pH<7.5, the feeding of CO₂was ceased. Natural sunlight was used for cultivation. The illuminationintensity at daytime was controlled up to 60000 lux. The OD₆₈₀ value ofthe microalgae suspension was detected every day. A continuouscultivation was conducted for 14 days. The growth curve of themicroalgae was showed in FIG. 3.

Example 7

The example was substantially same as example 6, except that 1.5 g/L ofsodium nitrate in the medium was replaced with 1.35 g/L of sodiumnitrite and 0.15 g/L of sodium nitrate. The growth curve of themicroalgae was showed in FIG. 3.

Example 8

The example was substantially same as example 7, except that themicroalgae cultivated was Monoraphidium sp. The growth curve of themicroalgae was showed in FIG. 3.

It could be seen from FIG. 3 that the microalgae strain selected couldgrow successfully using either nitrate or nitrite.

Examples 9-16 illustrated the impact of EM bacteria on the metabolism ofinorganic nitrogen source by microalgae, with the addition of a greatdeal of organic carbon source.

Example 9

A BG11 medium (having nutrient ingredients according to Table 1, withoutsterilization) was firstly used to cultivate Chlorella sp. When theOD₆₈₀ value reached 4, an amount of heterotrophic medium nutrientingredients as specified in Table 4 was supplemented once. Thetemperature was controlled between 20 and 30° C. Compressed air and CO₂were fed for cultivation. When the microalgae suspension had a pH>10,CO₂ was fed, while when the microalgae suspension had a pH<7.5, thefeeding of CO₂ was ceased. Natural sunlight was used for cultivation.The illumination intensity at daytime was controlled up to 60000 lux. 2g/L of glucose was added, and EM bacteria was added in an amount of2.9×10⁷ cells/L microalgae suspension. The OD₆₈₀ value of the microalgaesuspension was detected every day. After 1 day of cultivation, 10 g/L ofglucose was added again, and EM bacteria was supplemented in an amountof 3.6×10⁷ cells/L microalgae suspension. When the cultivation wasconducted to the fifth day, 10 g/L of glucose was supplemented again.During the cultivation, the bacteria count of the microalgae suspensionmonitored was up to 9.7×10⁶/ml microalgae suspension. Harvest was madeafter a 8 day continuous cultivation. After the last time of addingglucose, the feeding of CO₂ was ceased. At the terminal of thecultivation, the pH of the microalgae suspension was 8.6. A microalgaebiomass and a residual cultivation solution were obtained throughcentrifugal separation. An analysis of the residual cultivation solutionshowed a total content of NO₃ ⁻ and NO₂ ⁻ of <10 μg/g. The growth curveof the microalgae was showed in FIG. 4.

Example 10

The example was substantially same as example 9, except that themicroalgae cultivated was Monoraphidium sp. During the cultivation, thebacteria count of the microalgae suspension monitored was up to4.6×10⁷/ml microalgae suspension. At the terminal of the cultivation,the pH of the microalgae suspension had increased naturally to 8.2. Ananalysis of the residual cultivation solution showed a total content ofNO₃ ⁻ and NO₂ ⁻ of <200 μg/g. The growth curve of the microalgae wasshowed in FIG. 4.

Example 11

The example was substantially same as example 9, except for thefollowing aspects: the amount for the first addition of EM bacteriabeing 7.9×10⁷ cells/L microalgae suspension, without a second additionof EM bacteria; and the amount for the second addition of glucose being30 g/L, without a third addition of EM bacteria. During the cultivation,the bacteria count of the microalgae suspension monitored was up to2.6×10⁷/ml microalgae suspension. At the terminal of the cultivation,the pH of the microalgae suspension had increased naturally to 8.2. Ananalysis of the residual cultivation solution showed a total content ofNO₃ ⁻ and NO₂ ⁻ of <10 μg/g. The growth curve of the microalgae wasshowed in FIG. 4.

Example 12

The example was substantially same as example 11, except that themicroalgae cultivated was Monoraphidium sp. During the cultivation, thebacteria count of the microalgae suspension monitored was up to5.2×10⁷/ml microalgae suspension. At the terminal of the cultivation,the pH of the microalgae suspension had increased naturally to 7.8. Ananalysis of the residual cultivation solution showed a total content ofNO₃ ⁻ and NO₂ ⁻ of <200 μg/g. The growth curve of the microalgae wasshowed in FIG. 4.

Comparative Example 2

The example was substantially same as example 9, except that no EMbacteria was added. During the culture, the bacteria count of themicroalgae suspension monitored was up to 13.6×10⁸/ml microalgaesuspension. At the cultivation terminal, the pH of the microalgaesuspension had increased naturally to 7.2. The growth curve of themicroalgae was showed in FIG. 4.

It could be seen from FIG. 4 that the addition of EM bacteria promotedthe growth of microalgae and consumed rapidly the inorganic nitrogensource.

Example 13

A BG11 medium (having nutrient ingredients according to Table 1, withoutsterilization) was firstly used to cultivate Chlorella sp. When theOD₆₈₀ value reached 4, an amount of heterotrophic medium nutrientingredients as specified in Table 4 was supplemented once. Thetemperature was controlled between 20 and 30° C. Compressed air and CO₂were fed for cultivation. When the microalgae suspension had a pH>10,CO₂ was fed, while when the microalgae suspension had a pH<7.5, thefeeding of CO₂ was ceased. Natural sunlight was used for cultivation.The illumination intensity at daytime was controlled up to 60000 lux.Since inoculation of Chlorella sp., the cultivation was firstly made for2 days under autotrophic conditions by illumination. Then, 2 g/L ofglucose was added. EM bacteria was added in an amount of 1.8×10⁸ cells/Lmicroalgae suspension. The OD₆₈₀ value of the microalgae suspension wasdetected every day. After 3 day of cultivation, 10 g/L of glucose wasadded again, and EM bacteria was supplemented in an amount of 1.8×10⁸cells/L microalgae suspension. After 2 days of cultivation, 10 g/L ofglucose was supplemented again. During the cultivation, the bacteriacount of the microalgae suspension monitored was up to 2.9×10⁷ cells/mlmicroalgae suspension. Harvest was made after a 14 day continuouscultivation. After the last time of adding glucose, the feeding of CO₂was ceased. At the terminal of the cultivation, the pH of the microalgaesuspension was 9.2. A microalgae biomass and a residual cultivationsolution were obtained through centrifugal separation. An analysis ofthe residual cultivation solution showed a total content of NO₃ ⁻ andNO₂ ⁻ of <10 μg/g. The growth curve of the microalgae was showed in FIG.5.

Example 14

The example was substantially same as example 13, except for thefollowing aspects: the absence of a second addition of EM bacteria; andthe amount for the second addition of glucose being 30 g/L, without athird addition of EM bacteria. During the cultivation, the bacteriacount of the microalgae suspension monitored was up to 2.9×10⁷/mlmicroalgae suspension. At the terminal of the cultivation, the pH of themicroalgae suspension had increased naturally to 9.3. An analysis of theresidual cultivation solution showed a total content of NO₃ ⁻ and NO₂ ⁻of <10 μg/g. The growth curve of the microalgae was showed in FIG. 5.

Example 15

The example was substantially same as example 13, except that NaNO₃ inthe BG11 medium was replaced with KNO₃, and KNO₃ was added in an amountof 0.5 g/L. During the cultivation, the bacteria count of the microalgaesuspension monitored was up to 1.3×10⁷/ml microalgae suspension. At theterminal of the cultivation, the pH of the microalgae suspension was9.4. An analysis of the residual cultivation solution showed a totalcontent of NO₃ ⁻ and NO₂ ⁻ of <10 μg/g. The growth curve of themicroalgae was showed in FIG. 5.

Example 16

The example was substantially same as example 14, except that NaNO₃ inthe BG11 medium was replaced with KNO₃, and KNO₃ was added in an amountof 0.5 g/L. During the cultivation, the bacteria count of the microalgaesuspension monitored was up to 1.7×10⁷/ml microalgae suspension. At theterminal of the cultivation, the pH of the microalgae suspension was9.3. An analysis of the residual cultivation solution showed a totalcontent of NO₃ ⁻ and NO₂ ⁻ of <10 μg/g. The growth curve of themicroalgae was showed in FIG. 5.

It could be seen from FIG. 5 that using either potassium nitrate orsodium nitrate as the nitrogen source, the addition of EM bacteriapromoted the growth of microalgae.

Examples 17-18 illustrated the immobilization of NOx using a residualcultivation solution obtained from a microalgae cultivation and acontinued microalgae cultivation using the NOx immobilized solution.

Example 17

NOx was absorbed by the assistance of O₃.

A mixed gas of NO₂ and NO was used to simulate a practical flue gas. Acompressed air was used as carrier gas. The flow rate of NOx was 0.3L/min. A O₃-containing gas was provided by a Model XM-Y movable ozonizeravailable from Qingdao Xin Mei purification equipment Co., Ltd., with aflow rate of 1 L/min. Air was mixed to a total flow rate of 150 L/h. NOxconcentrations in the inlet and outlet gas outlet gases were measured. ANOx immobilizing ratio was calculated as:NOx immobilizing ratio=(1−NOx concentration at the outlet/NOxconcentration at the inlet)×100%;wherein the total concentration of NOx at the inlet was substantiallystable at 620 mg/m³ (with a NO content of about 600 mg/m³ and a NO₂content of about 20 mg/m³)

The flow chart was showed in FIG. 6. The absorption column had adiameter of 100 mm and a height of 700 mm. The column bottom wasequipped with a sieve gas distributor. 3 L of the residual cultivationsolution generated from example 16 was contained in the column. Duringoperation, a NOx mixed gas was fed directly into the absorption column.The operation was ceased after 22 h. The residual cultivation solutionwithin the column was taken out, and was measured for the total contentof NO₃ ⁻ and NO₂ ⁻ of 5900 μg/g.

Microalgae was cultivated using the NOx immobilized solution.

The NOx immobilized solution above was used as microalgae medium tocultivate Chlorella sp., where the other nutrient substances than thenitrogen source were provided referring to BG11 medium. The otherportions of the cultivation process were same as example 16. During thecultivation, the bacteria count of the microalgae suspension monitoredwas up to 1.8×10⁷/ml microalgae suspension. Harvest was made after a 14day continuous cultivation. After the last time of adding glucose, thefeeding of CO₂ was ceased. At the terminal of the cultivation, the pH ofthe microalgae suspension was 9.1. A microalgae biomass and a residualcultivation solution were obtained through centrifugal separation. Ananalysis of the residual cultivation solution showed a total content ofNO₃ ⁻ and NO₂ of <10 μg/g. It could be seen from FIG. 7 that using theNOx immobilized nutrient stream as a cultivation nutrient solution, thegrowth of microalgae was promoted after the addition of EM bacteria, bywhich NO₃ ⁻ and NO₂ ⁻ in the microalgae suspension was immobilized againand the microalgae suspension was reverted back to be alkaline, so as tobe further used as an alkaline immobilizing solution for the waste gasdenitration.

Example 18

The example was substantially same as example 17, except that 3 L ofresidual cultivation solution obtained from example 10 was contained inthe absorption column. After 22 h of immobilization, the residualcultivation solution in the column was taken out, which was measured tohave a total content of NO₃ ⁻ and NO₂ ⁻ of 5800 μg/g.

Microalgae was cultivated using the NOx immobilized solution.

The NOx immobilized solution above was used as microalgae medium tocultivate Monoraphidium sp., where the other nutrient substances thanthe nitrogen source were provided referring to BG11 medium. The otherportions of the cultivation process were same as example 10. During thecultivation, the bacteria count of the microalgae suspension monitoredwas up to 9.2×10⁶/ml microalgae suspension. Harvest was made after a 8day continuous cultivation. After the last time of adding glucose, thefeeding of CO₂-containing flue gas was ceased. At the terminal of thecultivation, the pH of the microalgae suspension was 8.7. A microalgaebiomass and a residual cultivation solution were obtained throughcentrifugal separation. An analysis of the residual cultivation solutionshowed a total content of NO₃ ⁻ and NO₂ ⁻ of <200 μg/g. It could be seenfrom FIG. 8 that using the NOx immobilized solution as a cultivationnutrient solution, the growth of microalgae was promoted after theaddition of EM bacteria, by which NO₃ ⁻ and NO₂ ⁻ in the microalgaesuspension was immobilized again and the microalgae suspension wasreverted back to be alkaline, so as to be further used as an alkalineimmobilizing solution for the waste gas denitration.

The example illustrated the impact of EM bacteria on the growth ofmicroalgae under heterotrophic conditions without light.

Example 19

The example was substantially same as example 9, except that themicroalgae cultivated under conditions without light. The pH of themicroalgae suspension at the terminal of cultivation was measured as7.7. The growth curve of the microalgae was showed in FIG. 9.

Comparative Example 3

The comparative example illustrated the nitrate assimilation by EMbacteria.

The present comparative example was substantially same as example 9,except that: only the cultivation of EM bacteria was conducted; themedium was sterilized before cultivation; the medium was still BG11(Table 1), whilst the initial concentration of NO₃ ⁻ was 6900 ug/g; andthe cultivation lasted for 14 days. An analysis at the cultivationterminal showed a total content of NO₃ ⁻ and NO₂ ⁻ of 5600 μg/g. Itcould be seen that, during the growth, EM bacteria consumed theinorganic nitrogen source with a much less rate than that of microalgae.

Example 20

The example illustrated the immobilization of NOx using an alkalineresidual cultivation solution.

3 L of the alkaline residual cultivation solution from example 14 wasanalyzed for the concentrations of potassium and sodium ions. 3 L of anaqueous solution having same potassium ion concentration and sodium ionconcentration was formulated, where the pairing anions were HCO₃ ⁻ andCO₃ ²⁻. The aqueous solution formulated had a pH of 9.27, substantiallysame as that of the alkaline residual cultivation solution from example14. The aforementioned alkaline residual cultivation solution and theformulated aqueous solution were used respectively as a immobilizingsolution to immobilize NOx using the process of example 17. A curveshowing the efficiency of immobilizing NOx was provided in FIG. 10.

It could be seen from FIG. 10 that the residual cultivation solution hada significantly higher efficiency of immobilizing NOx than that of theformulated alkaline solution.

Comparative Example 4

The comparative example illustrated the effect of cultivating Chlorellasp. with low concentration of NH₄HCO₃.

BG11 medium (Table 1) was used to cultivate Chlorella sp., while thenitrogen source in the BG11 medium was replaced with NH₄HCO₃ having aconcentration of 3.3 mmol/L, which was much less than that of the BG11medium (17.6 mmol/L). The initial concentration of the microalgaestrain, OD₆₈₀, was 0.5. Compressed air was fed for cultivation. Thetemperature was controlled between 20-30° C. During the culture, naturalsunlight was used for cultivation, and the illumination intensity ondaytime was controlled up to 60000 lux. The growth curve was provided inFIG. 11.

Comparative Example 5

The comparative example illustrated the effect of cultivating Chlorellasp. with low concentration of NaNO₃.

The comparative example was substantially same as comparative example 4,except that the nitrogen source in the medium was replaced with NaNO₃.The OD₆₈₀ value of the microalgae suspension was detected every day. Thegrowth curve was provided in FIG. 11.

Comparative Example 6

The comparative example illustrated the effect of cultivating Chlorellasp. with extra high concentration of NaNO₃.

The comparative example was substantially same as comparative example 4,except that the nitrogen source in the medium was replaced with NaNO₃,while the concentration of the nitrogen source was increased to 176mmol/L, which was much higher than that of the BG11 medium (17.6mmol/L). The OD₆₈₀ value of the microalgae suspension was detected everyday. The growth curve was provided in FIG. 11.

Example 20

The example illustrated the effect of an autotrophic culture ofChlorella sp. according to the present invention.

The example was substantially same as comparative example 4, except thatthe nitrogen source and the concentration thereof still followed theformulation of BG11 medium, and when the pH was higher than 10 duringthe late stage of the cultivation, nitric acid was supplemented toadjust the pH in an appropriate range. The OD₆₈₀ value of the microalgaesuspension was detected every day. The growth curve was provided in FIG.11.

Example 21

The example illustrated the effect of an autotrophic culture ofSpirulina sp. according to the present invention.

Z-medium (Table 3) was used to cultivate Spirulina sp. The microalgaestrain had an initial concentration, OD₆₈₀, of 0.3. Compressed air wasfed for cultivation. The temperature was controlled between 20-30° C.When the pH was higher than 10.5 during the late stage of thecultivation, nitric acid was supplemented to adjust the pH in anappropriate range. During the culture, natural sunlight was used forcultivation, and the illumination intensity on daytime was controlled upto 60000 lux. The OD₆₈₀ value of the microalgae suspension was detectedevery day. The growth curve was provided in FIG. 12.

Example 22

The example illustrated the effect of a mixotrophic cultivation ofChlorella sp. according to the present invention (withoutsterilization).

The example was substantially same as comparative example 4, except thata heterotrophic cultivation medium for Chlorella sp. (Table 4) was used.2 g/L of glucose and EM bacteria in an amount of 5×10⁷ cells/Lmicroalgae suspension were added every three days during the culture,and when the pH was higher than 10, nitric acid was supplemented toadjust the pH in an appropriate range. The OD₆₈₀ value of the microalgaesuspension was detected every day. The growth curve was provided in FIG.11.

Example 23

The example illustrated the effect of a mixotrophic cultivation ofSpirulina sp. according to the present invention (withoutsterilization).

The example was substantially same as example 21, except that 2 g/L ofglucose and EM bacteria in an amount of 5×10.5⁷ cells/L microalgaesuspension were added every three days during the culture, and when thepH was higher than 10.5, nitric acid was supplemented to adjust the pHin an appropriate range. The OD₆₈₀ value of the microalgae suspensionwas detected every day. The growth curve was provided in FIG. 12.

Example 24

The example illustrated the effect of an aseptic heterotrophiccultivation of Chlorella sp. according to the present invention.

The example was substantially same as comparative example 4, using aChlorella sp. heterotrophic medium (Table 4) for heterotrophiccultivation. The microalgae strain had an initial concentration, OD₆₈₀,of 0.5. Compressed air was fed. The culture was conducted under anaseptic state without light. The temperature was controlled between20-30° C. When the glucose was consumed substantially, 10 g/L of glucosewas added in time; while when the pH was higher than 10, nitric acid wassupplemented to adjust the pH in an appropriate range. The OD₆₈₀ valueof the microalgae suspension was detected every day. The growth curvewas provided in FIG. 11.

It could be seen from FIGS. 11-12 that the process according to thepresent invention increased the growth efficiency of microalgae. Ifabundant of nitrate was added at the initial stage of cultivation, thehigh concentration of nitrate would not promote significantly the growthof microalgae.

Example 25

The example illustrated the impact of the varied concentration of nitricacid or H₂O₂ on the hydrogen peroxide decomposition.

Nitric acid/H₂O₂ aqueous solutions with various concentrations wereformulated. After 10 days, the concentration of H₂O₂ was determined. Thedecomposition rate of H₂O₂ in the different concentrations of nitricacid/H₂O₂ aqueous solutions were accordingly calculated, resultsprovided in Table 6. (the concentration of hydrogen peroxide wasmeasured referring to the process of GB1616-2003)

TABLE 6 Hydrogen peroxide Nitric acid 6 wt % 3 wt % 0.3 wt % 35 wt % 25%  23% 19.3% 25 wt % 16% 15.5%   12% 15 wt %  9%  7.3%   5.4%

It could be seen from Table 6 that despite increasing the concentrationof nitric acid or increasing the concentration of hydrogen peroxide, thedissipation of hydrogen peroxide was increased significantly.

Example 26

The example illustrated the effect of the denitration of a lowconcentration of NOx according to the present invention.

A simulate waste gas was formulated by NO, NO₂ and nitrogen gas, with aNO concentration of 500 ppm (volume) and a NO₂ concentration of 20 ppm(volume). The absorption solution consisted of 15 m % of nitric acid,0.4 m % of hydrogen peroxide and balance of water. The absorption devicewas a glass column, having a diameter of 100 mm and a height of 700 mm.The glass column was equipped at the bottom with a sieve plate, having ahole diameter of 16 μm-30 μm. The column contained 3000 ml of absorptionsolution. The flow rate of the simulate waste gas was 150 L/h. The testwas carried out at room temperature under atmospheric pressure. The testresult was provided in Table 7. (referring to the process ofGB/T14642-2009, no nitrite was found in the absorption solution afterthe test)

TABLE 7 Treatment time/h 1 2 4 8 12 16 20 26 31 36 41 47 Outlet NO/ppm460 420 360 260 150 35 30 24 20 17 21 19 Outlet NO₂/ppm 1 0 0 0 0 5 1013 18 23 19 21 Outlet NOx/ppm 461 420 360 260 150 40 40 37 38 40 40 40

It could be seen from Table 7 that at the initial stage of denitration,the denitration activity of the absorption solution was very low. Thedenitration activity of the absorption solution increased gradually andsuccessively over time. 16 hours later, the denitration activity of theabsorption solution reached a stable stage, where the denitration ratiowas more than 90%.

Example 27

The example illustrated the effect of the denitration of a lowconcentration of NOx according to the present invention.

The example was substantially same as example 26, except that theconcentration of hydrogen peroxide was 1 m %, and the concentration ofnitric acid was 25 m %. The test result was provided in Table 8.(referring to the process of GB/T14642-2009, no nitrite was found in theabsorption solution after the test)

TABLE 8 Time/h 1 2 4 8 12 16 20 Outlet NO₂/ppm 0 0 0 0 0 2 11 OutletNOx/ppm 430 400 330 220 100 38 38

Example 28

The example illustrated the effect of the denitration of a highconcentration of NOx using a single column according to the presentinvention.

The example was substantially same as example 26, except that theconcentration of hydrogen peroxide was 0.3 m %, and the concentration ofnitric acid was 15 m %; and the simulate waste gas had a NOconcentration of 3200 ppm (volume) and a NO₂ concentration of 100 ppm(volume). The test result was provided in Table 9. (referring to theprocess of GB/T14642-2009, no nitrite was found in the absorptionsolution after the test)

TABLE 9 Time/h 1 2 4 8 12 16 20 24 30 35 40 45 Outlet NO/ppm 2310 19001600 1400 1300 1250 1200 1000 830 750 800 830 Outlet NO₂/ppm 60 50 35 3530 30 50 120 290 320 290 260 Outlet NOx/ppm 2370 1950 1635 1435 13301280 1250 1120 1110 1070 1090 1090

Comparative Example 7

The comparative example illustrated the effect of denitration using ahigh concentration of H₂O₂.

The example was substantially same as example 26, except that theconcentration of hydrogen peroxide was 2.5 m %, and the concentration ofnitric acid was 15 m %. The test result was provided in Table 10.

TABLE 10 Time/h 1 2 4 8 12 16 20 NO/ppm 59 20 50 30 25 25 35 NO₂/ppm 1425 15 20 20 15 10 NOx/ppm 73 45 75 50 45 40 35

Example 29

The example illustrated a process involving the acid procedure using thesystem according to the present invention.

Referring to FIG. 14, 150 L/h of a mixed gas comprising 480 ppm of NOand balance of air was firstly fed into denitration reactor 1-1(containing 0.5% aqueous hydrogen peroxide and 15% aqueous dilute nitricacid solution) for reaction to provide a dilute nitric acid. The yieldof nitric acid was 0.19 kg/h. The purified gas C after immobilizationwas vented.

3 kg of microalgae nutrient solution E was fed into NOx immobilizingnutrient stream formulating device 1-2 (the nutrient solution consistingof Z-medium +10 g/L NaNO₃), mixed homogeneously with residualcultivation solution F and dilute nitric acid, and fed into microalgaecultivation device 2, to which a concentration of microalgae strain Dwas added to provide a final microalgae suspension concentration ofOD=0.3. CO₂ with a concentration of 2% (by volume) was fed into themicroalgae cultivating device 2 at a flow rate of 200 L/h.

When the pH of the microalgae suspension <8.5, the feeding of CO₂ wasceased; while when the pH of the microalgae suspension >10.5, thefeeding of CO₂ was continued. The illumination intensity was 10000 lux.

After the completion of cultivation, the microalgae suspension was fedinto microalgae filter separator 3 for filtration and separation, and2.5 kg of residual cultivation solution F obtained therefrom wasreturned to NOx immobilizing nutrient stream formulating device 1-2 forrecycle cultivation. 250 g of concentrated microalgae biomass G was fedinto microalgae dryer 4 for drying, to provide 25 g of microalgaeproduct.

The invention claimed is:
 1. A process of cultivating microalgae,comprising: washing an industrial waste gas using an absorption solutioncomprising 0.5 mass % to 58 mass % of nitric acid and 0.001 mass % to 25mass % of hydrogen peroxide to obtain a purified gas and a NOximmobilized solution; forming a NOx immobilized nutrient streamcomprising the NOx immobilized solution and a residual cultivationsolution; preparing a microalgae suspension; adding the NOx immobilizednutrient stream to the microalgae suspension; adding effectivemicroorganisms (EM) to the microalgae suspension, wherein the EMcomprises photosynthetic bacteria, lactobacillus, yeast, gram-positiveactinomyces, and filamentous bacteria; growing microalgae in themicroalgae suspension; and separating the microalgae suspension into amicroalgae biomass and the residual cultivation solution, wherein theindustrial waste gas comprises NOx, and wherein the NOx immobilizednutrient stream comprises a nitrogen source that is NO³⁻, NO²⁻, or amixture thereof and an amount of nitrogen source, calculated as nitrogenatoms, is 10-300 mmol/L.
 2. The process according to claim 1, whereinthe microalgae is a heterotrophic or mixotrophic microalgae.
 3. Theprocess according to claim 2, wherein the microalgae is selected fromthe group consisting of Cyanophyta and Chlorophyta.
 4. The processaccording to claim 2, wherein the microalgae is Chlorella sp.,Scenedesmus sp., Monoraphidium sp. or Spirulina sp.
 5. The processaccording to claim 1, further comprising adding into the NOx immobilizednutrient stream an organic carbon source selected from the groupconsisting of sugar, organic acid, salt of an organic acid, alcohol,cellulose hydrolyzate, starch hydrolyzate, and mixtures thereof.
 6. Theprocess according to claim 5, wherein a concentration of the organiccarbon source is 1 g/L microalgae suspension to 30 g/L microalgaesuspension.
 7. The process according to claim 1, wherein the EM bacteriais added in an amount of 1×10⁵ cells/L microalgae suspension to 9×10⁸cells/L microalgae suspension.
 8. The process according to claim 1,wherein the microalgae suspension has a temperature of 15° C. to 40° C.and a pH of 6-11.
 9. The process according to claim 1, furthercomprising illuminating the microalgae suspension at an illuminationintensity of 1000-200000 lux.
 10. The process according to claim 1,wherein the nutrient stream comprises at least a nitrogen source, aphosphorus source, and a carbon source, wherein at least one of thenitrogen source, the phosphorus source, and the carbon source is in theform of an alkali nutrient salt, wherein, during the cultivation, the pHof the microalgae suspension is adjusted using nitric acid, nitrousacid, or a mixture thereof.
 11. The process according to claim 5,wherein the organic carbon source is selected from the group consistingof glucose, levulose, acetic acid, sodium acetate, lactic acid, ethanol,methanol, cellulose hydrolyzate, and mixtures thereof.
 12. The processaccording to claim 1, further comprising: separating the microalgaesuspension to obtain a residual cultivation solution and microalgae; andwashing an industrial waste gas using the residual cultivation solutionto obtain the NOx immobilized nutrient stream, wherein the industrialwaste gas contains NOx.
 13. The process according to claim 1, whereinthe amount of the nitrogen-containing compound in the NOx immobilizednutrient stream is 20-200 mmol/L.